What year, make and model of tandem are you riding and/or who's seat post are you using and what size is it. Note: The actual manufacturer's name is usually engraved in the seatpost just below the "max height" line along with the diameter. Also, how much seatpost is exposed between the top of your seat tube and the seat rail and how far up the post is the stoker's stem attached?
It has been my experience that most OEM seatposts are fairly heavy and stiff: 27.2mm models tend to use fairly thick aluminum alloy tubing. Kalloy is a popular OEM label and they make many of the other brand-labeled seat posts, e.g., Ritchey. Both of the standard level Kalloy and Ritchey seatposts (Comp) are fairly stiff. The 29.2mm models are inherently stiff due to their diameter; however, can still feel somewhat flexy since some really thin-walled tubing is often used, e.g., Zoom models used on Co-Motion's tandems.
The reason for asking the length of the exposed seatpost and location of the stoker's stem is pretty obvious. The more seatpost you expose the more it will flex and the higher the stoker's stem is mounted the more the stoker's movements will contribute to any seatpost flex. So, to respond to your last comment, yes -- the captain's seatpost does pull double-duty as both a seat post and as a structural, load bearing frame extension and therefore if often times not ideally suited for lightweight seatposts.
We've been experimenting with titanium and carbon seatposts and they definitely are more elastic than the Ritchey Comp posts they replaced. The jury is still out on both. Titantium seems to be a very good material IF you don't mind a little flex. The carbon posts (Easton EA-70 models) are noticably more compliant than the Titantium. For the stoker, both of the materials seem to offer a very nice alternative to a suspension post in that they have some nice natural shock absorbing qualities and trim off excess weight (the 350mm Ritchey posts weigh 330grams, the Easton 350mm models are 210g and the USE Alien Ti 350mm model is about 180g. Perhaps the nicest seatpost made for tandem use is the Thomson models; however, their set-back seat post design is awkward for use as a Captain's post since the bend in the shaft is placed right where a stoker's stem would logically be attached.
Anyway, if you can provide some additional info on your tandem and posts that would be helpful.